OTTAWA — Three people were killed and 23 injured when a double-decker city bus struck a transit shelter in Ottawa on Friday afternoon, badly damaging the vehicle’s upper deck where several people were trapped inside.

The driver of the bus was arrested at the scene, Chief Charles Bordeleau of the Ottawa police told an evening news conference at city hall. He wouldn’t say why she was detained rather than submitting to questions voluntarily.

“Something led us to having to arrest the individual and take them to the station,” Bordeleau said.

The crash took place west of downtown at 3:50 p.m., just as dusk was beginning to settle on the coldest day of the winter so far in the capital. The bus was headed to Kanata, a more distant Ottawa suburb.

Mayor Jim Watson said two of the dead were riding the bus and one was on the platform at Westboro station, a major stop on the city’s busway.

The vehicle apparently jumped the curb as it approached the station and plowed along the platform, where the roof of a shelter carved deep into the vehicle’s second deck. The first seats on the upper level of the bus were crushed together.

During the emergency response, a dozen ambulances lined one side of the transit route, waiting to take injured people away.

One woman who could walk was helped off a bus by two first responders, one supporting each of her arms. Another was wheeled on a gurney in obvious distress, wailing loudly enough to be heard on an overpass above.

Neither Bordeleau nor Watson would speculate on why the bus had left the road. Despite mid-afternoon temperatures of about minus-15, it hadn’t snowed in Ottawa Friday and the busway is the top priority for the city’s snowplows, salters and sanders.

“We’re taking every necessary step to establish all of the facts and will co-operate fully with any investigation,” said John Manconi, the general manager of Ottawa’s transit service, OC Transpo.

Paramedics said a total of 25 people were taken to hospital, suggesting one of the victims could not be revived at the scene and the other two died after arriving. Of those transported, 14 were in critical condition and 11 were in serious condition, paramedic chief Myles Cassidy said in the news conference later.

“Most of the serious injuries were on the upper-right side of the bus,” Bordeleau said. “I want to express my sorrow and condolences of the Ottawa police service and other emergency services.”

He said the upper deck of the bus was badly damaged in the crash, trapping several people inside for some time.

The investigation will take a long time and likely would mean the closure of the busway and nearby roads for at least many hours, Bordeleau said.

Gabe Rivett-Carnac arrived at Westboro station on a double-decker bus himself about an hour before the crash. He was in his apartment across the street from the station when he started hearing sirens just after 4 p.m.

He went outside and came across a “chaotic” scene, with at least 30 emergency vehicles, including seven or eight fire trucks, and lots of police and bystanders, he said.

“You could see the entire front portion of the bus had been demolished or cut away, and firefighters were on ladders cutting out the top floor windows,” he said. “I saw one person being lowered from the top floor in a seated position.”

He said he could tell there would be deaths given the devastation at the scene.

“Based on the way it looked, if there was anybody at the front of the bus at the top â¦ I can’t see how they would’ve been able to get away from that,” he said.

His partner usually gets off the bus at that station when coming home from work but she hadn’t gone in on Friday, he said, and they were checking in with family and friends who live in the area to see if they’re OK.

“You get a knot in the pit of your stomach. I felt like vomiting a couple of times. It’s not a comfortable thing to witness and I can’t imagine experiencing it.”

“To the people of Ottawa, know that the rest of Ontario shares in your sorrow tonight. We grieve with you, and we stand with you,” Ford wrote in a longer statement.

“Such devastating news. My thoughts and condolences are with the victims, their families and all those affected by this tragedy,” wrote Environment Minister Catherine McKenna, who represents the area in Parliament.

In 2013, another Ottawa double-decker bus broke through a warning gate and collided with a Via Rail train in suburban Barrhaven, killing six people as the train sheared off the front of the vehicle. In that case, an investigation found that the cause was a combination of excessive speed, a difficult curve before the tracks and driver distraction from video monitors showing the upper deck.